Here is a screen capture of a real help wanted ad for an administrative assistant posted on New York City Craigslist by Sotheby's. The ad has been fixed now, presumably not by its original poster (click on the magnifying glass at the bottom to enlarge)

Update: it's fake. A Sotheby's spokeswoman emails:

We can confirm that it is not a Sotheby’s job posting. We don’t hire for a position of that title, and we don’t use Craigslist as a recruiting tool.

We have also checked with our colleagues at Sotheby’s International Realty and they confirmed it wasn’t them either. Our guess is that this is some kind of a hoax.